1. Field of the Invention
The present invention relates to a transmission for an automotive vehicle and more particularly to a 4-speed transmission including an overdrive.
2. Description of the Prior Art
According to U.S. Pat. No. 2,725,762, a transmission using a compound planetary gear set to provide four forward speed ratios and one reverse speed ratio is known. The compound planetary gear set comprises a first sun gear, a second sun gear, a group of long planetary pinions which mesh with the first sun gear, a group of short planetary pinions which mesh with the second sun gear and a ring gear. A pinion carrier has two planetary shafts for rotatably mounting the long and short pinions. This compound planetary gear set, therefore, has four rotary elements.
The transmission comprises a drive shaft, a driven shaft and a hydraulic torque converter having an impeller connected to the drive shaft, a hollow turbine shaft connected to the turbine of the torque converter, and an intermediate shaft connected to the drive shaft.
One of the four rotary elements of the planetary gear set, that is, the ring gear, is connected permanently to the driven shaft, but the other three rotary elements thereof are not permanently connected to any of the other rotary shafts, that is, the turbine and intermediate shafts. Instead of providing the permanent connection, three clutches are associated with these three rotary elements. The sun gears are connected with the turbine shaft through clutches, respectively, and the carrier is connected with the intermediate shaft through a third clutch. Two brakes when engaged hold the first sun gear and the carrier, respectively.
If the clutches and brakes are operated in the following manner, this known transmission provides a first speed (low), a second speed (intermediate), a third speed (direct drive) and a fourth speed (overdrive).
In the first, second and third speeds, turbine power is fed to the second sun gear. The carrier is held during the first speed operation, and the first sun gear is held during the second speed operation. The third or direct drive is obtained when the third clutch is also engaged to connect the carrier with the intermediate shaft which is permanenetly connected to the drive shaft bypassing the torque converter. To obtain the fourth speed or overdrive, the clutch associated with the second sun gear is disengaged and the first sun gear is held by the associated brake with the third clutch kept engaged. During the overdrive operation, the second sun gear, together with its associated intermediate shaft, free wheels at an excessively high speed.
If a ratio (.alpha..sub.1) of the number of teeth of the ring gear to the number of teeth of the first sun gear is assumed to be 0.5, and a ratio (.alpha..sub.2) of the number of teeth of the ring gear to the number of teeth of the second sun gear is assumed to be 0.417, the rotational speed of the second sun gear can be expressed as (1+.alpha.).times.N where N is the rotational speed of the drive shaft. Thus, the second sun gear free wheels at a speed approximately 2.2 times that of the drive shaft and overspeeds the driven shaft during the overdrive operation. This means that when the drive shaft rotates at 6,000 rev./min, the second sun gear rotates at 13,200 rev./min.
Therefore, this transmission tends to experience vibration and durability problems because the second sun gear rotates at an excessively high speed in overdrive.
Another known transmission disclosed in Japanese patent specification (Tokkaisho) No. 51-9092 provides four speed ratios forward and one speed ratio reverse without causing any rotary element to rotate at excessively high speed. The transmission includes three simple planetary gear sets connected in tandem.
This known transmission is disadvantageous with respect to its weight, space and cost because three planetary gear sets have to be used.
An object of the present invention is therefore to provide a transmission which provides four forward speed ratios including an overdrive and one reverse speed ratio and having only two planetary gear sets, so that it has less weight and saves space and cost, and which has no rotary element that rotates at excessively higher speeds than the drive shaft, so that it is vibration free and durable over long use.